Material properties and specifications associated with tank heads used on rail tank cars are directed by the Association of American Railroads (“AAR”) under AAR Specification M-1002 entitled “AAR Manual of Standards and Recommended Practices, Section C-Part III, Specification for Tank Cars.” AAR Specification M-1002 is governed by DOT 173.31(f), which states:                (f) Special requirements for hazardous substances. (1) A tank car used for a hazardous substance listed in paragraph (f)(2) of this section must have a tank test pressure of at least 13.8 Bar (200 psig), head protection and a metal jacket, except that—                    (i) No metal jacket is required if—                            (A) The tank test pressure is 23.4 Bar (340 psig) or higher; or                (B) The tank shell and heads are manufactured from AAR steel specification TC-128, normalized; . . .                                                
AAR Specification M-1002 is also governed by DOT 179.100-8(b), which states: “Each tank head made from steel which is required to be ‘fine grain’ by the material specification, which is hot formed at a temperature exceeding 1700° F., must be normalized after forming by heating to a temperature between 1550° and 1700° F., by holding at that temperature for at least 1 hour per inch of thickness (30-minute minimum), and then by cooling in air.” The purpose of the normalizing heat treat practice is to ensure that the tank head has the impact toughness properties addressed in AAR M-1002, section 2.2.1.2, which requires:                Effective for cars ordered after Aug. 1, 2005, each plate-as-rolled of ASTM A516, A302, A537, and AAR TC128 steel used for pressure tank car heads and shells must be Charpy impact tested transverse to the rolling direction in accordance with ASTM A20. The test coupon must simulate the in-service condition of the material and must meet the minimum requirement of 15 ft-lb average for three specimens, with no single value below 10 ft-lb and no two below 15 ft-lb at −30° F. Plates for low temperature service described in 49 CFR 179.102 that require longitudinal impact testing at −50° F. do not require transverse testing at −30° F.        
As is clear from DOT 179.100-8(b), it is industry practice to hot form railroad car tank heads. Hot forming typically involves heating a circular steel plate blank in an oven which may be above the normalization temperature, and pressing the hot steel blank in a hydraulically powered press to form an ellipsoidal tank head. This process is expensive in terms of equipment and is time consuming. The AAR standards do not contemplate or address tank car heads fabricated through a cold forming process and then heat treated after cold forming.
The impact toughness of tank heads for rail cars is of vital importance, as demonstrated by recent tragic accidents in Lac-Mégantic, Quebec and Casselton, N. Dak. Lac-Mégantic was the site of a train derailment in July of 2013 that killed forty-seven people. In that incident, a freight train with seventy-two tank cars filled with crude oil ran away and derailed, resulting in the fire and explosion of multiple tank cars near the town's center. In addition to the casualties, more than thirty buildings were destroyed. Just outside of Casselton, a train carrying crude oil struck wreckage from a prior derailment on Dec. 30, 2013, igniting the crude oil and causing a chain of large explosions which were heard and felt several miles away. Authorities issued a voluntary evacuation of the city and surrounding area as a precaution. The crash occurred in proximity to a populated area, and it was fortunate that no casualties resulted.
Prior methods of cold-forming tank car heads have involved a one stage cold-forming step wherein a high-force hydraulic press (e.g. a 12,000 ton hydraulic press) is operated to cold-form a steel blank into an ellipsoidal tank head by one pressure stroke or a few pressure strokes. These methods were attempted in the 1960s and earlier.
One drawback of early cold-forming approaches is that the equipment was limited to a single tank car head size specification. In order to adapt the forming equipment to manufacture a variety of tank car head sizes, a corresponding variety of dies had to be provided at high expense. Changing the set-up of the press equipment from one tank head size to another added further time and expense.
More importantly, the use of brute force to cold-form a tank car head in a very short period of time may cause material damage and introduce significant stresses in the material. Where the steel blank is over ⅜ of an inch thick, finite cracks are highly suspect in rapid cold-forming operations. Thus, rapidly cold-formed tank car heads have in the past required very careful and time-consuming inspection.
It is believed that the equipment requirements, inspection demands and quality concerns associated with rapid single stage cold-forming methods of the prior art have more than negated the benefits of faster production, thereby leading to the current acceptance of hot-forming as the industry standard for tank car head production.
Thus, there has long been a need for an improved cold-forming process for making tank car heads that avoids the drawbacks of earlier cold-forming processes. The need for an improved manufacturing process has grown urgent in view of safety concerns raised by recent accidents, including the highly publicized accidents in Lac-Mégantic and near Casselton.